Tuner concentration control apparatus

ABSTRACT

An apparatus adapted for use in an electrographic magnetic brush development station for maintaining the concentration of toner in the developer mixture at a substantially constant level. Toner concentration is monitored by sampling the self-biasing potential generated by the brush as it periodically contacts uncharged areas of the electrographic recording element. Such potential has been found to be inversely proportional to the concentration of toner in the developer mixture.

United States Patent Trachienberg et al.

TUNER CONCENTRATION CONTROL APPARATUS Inventors: William Trachienberg;Theodore H.

Morse, both of Rochester, NY.

Eastman Kodak Rochester, NY.

Filed: Sept. 3, 1971 Appl. No.: 177,600

Assignee: Company,

US. Cl. ..l18/7,1l7/17.5, 118/9, 118/637, 355/14 Int. Cl. ..B05c 11/00,003g 13/00 Field of Search .....ll8/4, 7, 8, 9, 637; 117/17.5, 117/934A; 355/3, 14; 346/74 References Cited UNITED STATES PATENTS Shelffo eta1. ..1l7/l7.5 Piper et al. ..1 18/2 1 March 6, 1973 3,599,605 8/1971Ralston ..1l8/637 3,529,546 9/1970 Kollar 101/426 3,399,652 9/1968Gawron.... ..l18/637 3,674,532 7/1972 Morse ..l l7/l7.5

Primary ExaminerMervin Stein Assistant Examiner--Leo MillsteinAttorney-Robert W. Hampton et a1.

[57] ABSTRACT An apparatus adapted for use in an electrographic magneticbrush development station for maintaining the concentration of toner inthe developer mixture at a substantially constant level. Tonerconcentration is monitored by sampling the self-biasing potentialgenerated by the brush as it periodically contacts uncharged areas ofthe electrographic recording element. Such potential has been found tobe inversely proportional to the concentration of toner in the developermixture.

4 Claims, 4 Drawing Figures PAPER FEED/N6 CHARGING PATENTEUH R ems SHEET10F 3 QERNQMK PATENTED 6|973 3,719,165 SHEET 2 BF 3 47 TONERREPLEA/(SHE? DELAY I 43 y FIG: 2 Q a THRESl-IOLD r L SAMPLE? SENSOR l Lz 42 45 SHIFT REGISTER x t lb I 47 Reset ran/ER 3/ 40 NTER REPLE/V/SHERliese/ 42 b l a //v THRESHOLD g SAMPLE? AMP SENSOR SH/FT REG/5m? WILL/AMTRACHTE/VBERG 3 THEODORE H. MORSE INVENTORS AGENT M TUNER CONCENTRATIONCONTROL APPARATUS BACKGROUND OF THE INVENTION This invention relates toelectrographic development and more particularly to an apparatus formaintaining the concentration of toner particles in a magnetic brushdevelopment station at the substantially constant level required forproducing copies of uniformly good density.

Electrographic developers commonly comprise a mixture of suitablypigmented or dyed resin-based particles, known as toner, and a granularcarrier material which functions to carry toner by generating triboelecvtric charges thereon. In developing electrostatic charge patternsproduced on the surface of an electrographic recording element, it iscommon to apply the developer to the charge pattem-bearing surface bycontacting such surface with one or more rotating magnetic developmentbrushes, the bristles of which comprise toner-coated metallic carrierparticles. Upon contacting the surface of the recording element, thetoner particles, which are charged to a polarity such as to be attracted to the electrostatic charge pattern, are separated from thecarrier particles by the stronger electrostatic forces and selectively.deposited on the surface in accordance with the charge pattern.Obviously, as successive electrostatic charge patterns are developed,there is a gradual depletion of toner from the development mixture,requiring subsequent toner replenishment to. avoid a gradual reductionin density of the developed or toned images.

To maintain the concentration of toner in the development brush at thelevel required for high density copies, it has been common heretofore torequire the operator of an electrographic copier to periodically obttisaon serve the reproduction quality and to replenish the I developmentbrush with additional toner particles when the legibility or density ofthe developed images dropped below a subjective acceptable level. Whilesuch a manual toner replenishment system is satisfactory for manyapplications, such a system has proven highly unsatisfactory when a highthroughput application exists.

Although more sophisticated automatic toner replenishment systems havebeen proposed for maintaining the toner concentration at an acceptablelevel, no such system has proven entirely satisfactory 'to date.Generally, conventional automatic toner control systemshave provenunreliable over extended periods of use, insensitive to slightvariations .in toner concentration of a magnitude adversely affectingimage quality, costly to fabricate, anddifficult because of spacelimitations to incorporate in electrographic copiers. Moreover, mostautomaticsystems are adapted to sample the toner concentration of only arelativelysmall portion of the entire development mixture; thus, .whenthe toner concentration in such sampled portiondoes not accuratelyreflect the average toner concentration in the entire mixture, over orunder-replenishment of toner results.

BRIEF SUMMARY OF THE INVENTION Still another object of the invention isto provide an improved automatic toner control apparatus which can bereadily incorporated in conventional magnetic brush developmentstations.

Yet another Object of the invention is to monitor the average tonerconcentration in the entire electrographic developer rnix carried by amagnetic brush applicator and to replenish the mix with toner when theconcentration thereof falls below an optimum level.

Briefly, such objects are achieved in accordance with the presentinvention by periodically sampling the elec tric potential of a magneticdevelopment brush, connected to ground through a high resistance, whenthe brush is in contact with a uncharged portion (i.e.,a non-chargepattern-bearing portion) of the electrographic recording element. It hasbeen found that the triboelectric interaction between the developmentmix carried by the magnetic development brush and they recording elementcauses charge to accumulate on the housing of the brush assembly. Theintensity of such charge has been found to be inversely proportional tothe concentration of toner in the development mix carried by themagnetic'development brush. By sampling the housing potential only whenan uncharged portion of the recording element passes the developingstation, the variable effect on the housing potential produced by thecharge pattern is eliminated. The sampled potential is fed to athreshold sensing circuit which, when the input thereto exceeds apredetermined level indicative of a drop in toner concentration below anacceptable level, transmits anelectrical signal whereby a conventionaltoner replenisher can be activated.

In addition to the above objects, other objects of the invention and itsvarious advantages will become apparent from the ensuing detaileddescription of preferred embodiments, reference being made to theaccompanying drawings in which like characters denotelike parts and inwhich:

BRIEFDESCRIPTION OF THE DRAWINGS -toner concentration monitoringapparatus in accordance-with a preferred embodiment of the invention;

FIG. 3 is a schematic illustration of an alternative tonerconcentrationmonitoring apparatus embodied by the invention; and

FIG. 4 is a waveform diagram illustrating the output of the sampling andintegrating components comprising the alternative embodiment of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS For a generalunderstanding of an electrographic reproduction apparatus wherein theinvention has particular utility, reference is made to FIG. 1 whereinvarious components of an electrophotographic copier are schematicallyillustrated. As in most electrophotographic copies, an endlessphotoconductive recording element is advanced along a predetermined pathadjacent to which are disposed the various electrophotographicprocessing stations which serve to form a toner image of the originaldocument on the surface of such recording element. As shown in FIG. 1',the recording element may be in the form of a flexible belt 2 comprisinga photoconductive. film 3 disposed on a conductive backing 4. Thephotoconductive film may comprise, for instance, a heterogeneous mixtureof a bisphenol- A-polycarbonate binder, a triarylmethane organicphotoconductor and pyrylium sensitizing dye. Belt 2 is driven and guidedalong a path in the direction indicated by the arrows past theprocessing stations by rollers 5, 6, 7, 8 and 9, roller being driven bymotor 10. As belt 2 passes charging station 11, a portion of itsphotoconductive surface receives a uniform electrostatic charge'from acorona source, or the like, which charges the entire width of the belt.As the belt continues to advance, the uniformly charged portion passesexposure station 12 where it is imagewise exposed to actinic radiationin accordance with the indicia on the original document. Such imagewiseexposure serves to selectively dissipate theuniform charge on thephotoconductive surface to form'adevelopable electrostatic chargepattern thereon corresponding to the indicia on the original.Development of the electrostatic charge pattern is accomplished as theportion of belt 2 bearing such pattern is advanced past developmentstation 13. The latter generally comprises a reservoir for containing anelectrographic developer and a magnetic development brush for applyingthe developer to the electrostatic charge pattern to render it visible.As previously mentioned, the development mixture comprises, in general,a mixture of electroscopic toner particles charged to a polarity so asto be attracted to the electrostatic charge pattern, and granularcarrier particles which carry the toner by generating triboelectriccharges thereon. Upon contacting the charge pattern,.the triboelectricbonds between the toner and carrier particles are overcome by thestronger electrostatic attraction between the toner particles and thecharge pattern, causing the toner particles to be separated from thecarrier and deposited on the photoconductive surface of belt 2 inaccordance with the charge pattern.

To reuse that portion of the photoconductive film bearing the developedor toner image, the toner image may be transferred to a receiving sheet14 on which it can be subsequently permanently fused. Such a transferis'commonly effectedby a sheet feeding device 15 which feeds receivingsheets, usually paper, film or other web material, from a, supplystation 16 to a transfer station 17 simultaneously with the passagetherepast of the toner bearing belt 2. A shift register R, disclosed inthe commonly assigned copending U.S. Pat. application Ser. N01 19,999,serves to control the timing of the electrophotographic operations andto synchronize the feeding of the receiving sheets with the movement ofthe photoconductive belt. The shift register R includes a rotatablesegmented and slotted cylinder 18 which is driven by suitable means,such as belt 19 extending from a pulley around roller 5 so that movementof cylinder 18 is in direct response to movement of the photoconductivebelt 2.

Transfer station 17 commonly comprises means for electrostaticallycharging the receiving sheet so as to attract the toner particles fromthe belt thereto, After the toner image is transferred to the receivingsheet, the sheet-is peeled away from the belt as the latter passes oversmall roller 7. The toner-bearing receiving sheet is then attracted byan endless mesh belt transport 20, traveling about rollers 21 in aclockwise direction and at the same speed as belt 2, and is advancedthereby past a fusing station 22 where the toner image is permanentizedby heat or the like. The receiving sheet with its toner-bearing surfacefacing downward is caused to adhere to transport 20 by a source ofnegative pressure on the rear surface of the lower leg of the transport.After fusing, the receiving sheet is dropped into a receptacle 23.

Referring now to FIG. 2, developing station 13 comprises a conventionalmagnetic development brush 31 which is journaled for rotation in atrough 34 containing the electrographic developer mix 35. Means areprovided for rotatably driving the magnetic brush in a counterclockwisedirection toward the approaching belt 2. The development brush generallycomprises a magnetized metallic cylinder to which toner-coatedmagnetically attractible carrier particles, comprising the developer mix35, are attracted in chainlike arrangements to simulate the bristles ofa brush. Trough 34 is arranged adjacent the path along which belt 2travels, being spaced therefrom such that the bristles of thedevelopment brush contact the photoconductive surface of belt 2 at alltimes.

As previously mentioned, only a portion of the photoconductive surfaceof belt 2 receives a uniform electrostatic charge as it passes chargingstation 11. Shift register R serves to activate the charging stationonly for a predetermined time period, sufficient to uniformly charge asector of belt 2 long enough to receive an image of the longest originaldocument which the copier is designed to handle. After each chargingperiod, the charging station is inactivated and no charge is applied tothe belt surface for a predetermined time period, usually a timesufficient for a few inches of the belt to pass by. Thus, as belt 2approaches developing station 13, after being imagewise exposed atexposing station 12, its photoconductive surface bears equally spaceddevelopable charge patterns 36, shown as a negative charge, betweenwhich uncharged areas 37, bearing virtually no charge whatsoever arepresent.

Now, in accordance with the present invention, it has been found that,as a magnetic development brush 31 which is connected to ground throughsome resistance R continuously rotates against, and thereby interactswith, the photoconductive surface of belt 2, an electrical potential isgenerated on the development brush. Moreover, it has been found that thetwo major factors which determine the magnitude of such potential arethe concentration of toner in the developer and the charge on thatportion of the belts surface contacting the brush. Toner concentrationaffects the potential of the brush due to the phenomenon oftriboelectrification. As in the case of the electrographic developer,wherein two dissimilar materials are mixed together and become chargedrelative to one another, the surface of belt 2 and the development brushcomprise such dissimilar materials which, when rubbed-together, asoccurs when the rotating brush contacts the moving belt surface,generate triboelectric charges of opposite polarity on each other. Sincethe magnitude of the generated triboelectric charge is dependent on thecomposition of the materials, such composition determining the positionof the materials in the triboelectric series, and since the compositionof the development brush is a function of the toner concentration in thedeveloper carried by such brush, the triboelectric charge generated onthe brush is a function of the toner concentration in the brush.Actually, the brush potential has been found to be directly proportionalto the carrier concentration, hence, inversely proportional to tonerconcentration. Since the composition of belt 2 is substantiallyconstant, it has no variable effect on the brush potential. The chargepattern borne by the surface of belt 2 affects the brush potential dueto induction. Since the charge pattern varies in intensity, it producesan undesirable variable effect on brush potential; To eliminate sucheffect, means are provided for sampling the brush potential only whenthe brush is contacting the uncharged portion of the belt betweensuccessive charge patterns. In this manner, there is no need tocompensate for the variable effect on .brush potential produced by thecharge pattern.

As shown in FIG. 2, an electrical conductor 40 is coupled to themagnetic brush 31 to sense the potential thereof. Actually, the brushpotential can be sampled by connecting conductor 40 to trough 34,assuming it is electrically conductive, since the metallic carrierparticles serve to electrically couple the brush and the trough. Thedevelopment brush 3.1 is connected via conductor 40 to ground through ahigh resistance R,,, the value of which is selected so as to enhance thevariability of brush potential. Typically, R is approximately 100megohms, The brush potential sensed is amplified by a high inputimpedance operational amplifier 42 and passed through a sampling circuit43 comprising a normally open switch which may be periodically closed bythe shift register R for a short period during which the developmentbrush is in contact with a non-charged portion 37 of the belt surface.The pulsed output a of the sampler is then fed to a conventionalthreshold sensor 45 which transmits a signal b when the amplitude of anypulse received thereby ex ceeds a predetermined threshold level. Theoutput of the threshold sensor is then used to activate a conventionaltoner replenisher 47 which adds toner to the developing station whenactivated. To allow the added toner to blend in with the developercontained in trough 34 to form a substantially homogeneous mix, a simpledelay circuit 48 is coupled with the output of the threshold sensor tooverride the sampling circuit for a predetermined time.

In FIG. 3, alternative signal processing circuitry is shown whichincludes an integrating operational amplifier 55. The integratingamplifier is used as a safeguard to prevent transients, noise or otherelectrical disturbances which might ocassionally affect the amplitude ofthe pulsed output a of sampler 43 from activating the toner replenisherwhen additional toner is not, in fact, required. As shown in FIG. 4,wherein the outputs of the sampler, integrating amplifier and thresholdsensor are shown as a function of time, the integrating amplifier output0 will gradually increase as positive-going pulses are applied to itsinput. Since the output of the integrating amplifier does not return tozero during the period sampling is not taking place, 2

means must be provided for periodically dumping the voltage accumulatedby the amplifier. To accomplish this function, a conventional countingcircuit 56 is provided which automatically dumps or resets the voltageaccumulated by the integrating amplifier to zero volts after apredetermined number of pulses is counted. If the integrator outputexceeds the threshold of sensor 45 before the counting circuit resetsthe integrator amplifier, the output of the threshold sensor can be usedto reset the counter to zero.

EXAMPLE 1 A positively charged toner comprising carbon black, nigrosinebase, docosanoic acid, poly(4,4'-isopropylidene diphenyl-alt-ethylenecarbonate). was mixed with an electrographic carrier at a concentrationof approximately 5.5 percent toner-to-carrier by weight to form adeveloper mixture. The carrier consisted of l-Ioeganaes EH sponge ironparticles sieved +150, then nickeled to 1 percent by weight. Thedeveloper mixture was then used in a magnetic brush developing stationto produce copies from various original documents. The level ofpotential of the magnetic brush was sampled before and after each copy.It was found during this period that the brush potential rose from -40to -78 volts. The toner concentration was determined, after the copieswere made, to have been reduced to approximately 4.8 percent.

EXAMPLE 2 The developer of Example 11 was manually replenished to about6 percent toner-to-carrier by weight, and another process cycle of 29copies was made from a single original. The brush potential during thisperiod rose from -25 to -40 volts. The toner concentration wasdetermined to have been reduced to 5.5 percent. The entire system waspermitted to rest for 10 minutes. A series of seven copies was run andthe brush potential initially registered -40 and then climbed to -45volts.

EXAMPLE 3 A fresh developer and photoconductive belt combination wastested in the manner described in Example 1. Using a single originalwhich had few solid areas, the potential rose -12 volts over a processcycle of 86 prints.

EXAMPLE 4 Example 3 was repeated, except that an original with anaverage amount of solid areas was used. In two separate process cycles,it was found that the potential rose -17 volts after 84 prints and -18volts after 92 prints, respectively. Between the two cycles, 10 gm(approximately 0.4 percent) of toner was used to manually replenish thedeveloper. It was noted that the potential dropped l 5 volts afterreplenishment.

From the foregoing examples, it is apparent that the concentration oftoner in the developer carried by the brush is inversely proportional tothe brush potential, the lower the toner concentration, the higher thebrush potential.

This invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:

1. In an electrographic copier including means for advancing anelectrographic recording element bearing spaced electrostatic chargepatterns on a surface thereof past a rotatably mounted magneticdevelopment brush carrying a developer mixture comprising toner andmagnetically attractable carrier particles, such brush beingelectrically connected to ground potential through a resistor ofsubstantial resistance and arranged to continuously contact the surfaceof the recording element as such element advances along thepredetermined path, the improvement comprising:

apparatus for maintaining substantially constant concentration of tonerparticles in the developer mixture carried by the brush, said apparatuscomprising means, electrically coupled with the magnetic developmentbrush, for sampling the electrical potential of the brush as thedeveloper mixture carried by the brush periodically contacts portions ofthe recording element between successive charge patterns, said potentialresulting, at least in part, from the triboelectric interaction betweenthe recording element and the developer mixture and being inverselyproportional to the concentration of toner particles in the developermixture.

2. The invention accordance to claim 1 wherein said apparatus furthercomprises means operatively coupled to said sampling means foractivating a toner replenishing device when the sampled potentialexceeds a predetermined threshold level.

3. The invention according to claim 1 further comprising an integratingmeans operatively coupled with said sampling means for accumulating theelectrical potential sampled thereby, and means for activating a tonerreplenishing device when the potential accumulated by said integratingmeans exceeds a predefined threshold level.

4. The invention according to claim 3 further comprising counting means,operatively coupled with said sampling means, for counting the number ofregions on the belt surface sampled by said sampling means and forresetting the potential accumulated by said integrating means to zero inthe event the accumulated potential fails to exceed said predefinedthreshold level before said counting means counts a predetermined numberof regions.

1. In an electrographic copier including means for advancing anelectrographic recording element bearing spaced electrostatic chargepatterns on a surface thereof past a rotatably mounted magneticdevelopment brush carrying a developer mixture comprising toner andmagnetically attractable carrier particles, such brush beingelectrically connected to ground potential through a resistor ofsubstantial resistance and arranged to continuously contact the surfaceof the recording element as such element advances along thepredetermined path, the improvement comprising: apparatus formaintaining substantially constant concentration of toner particles inthe developer mixture carried by the brush, said apparatus comprisingmeans, electrically coupled with the magnetic development brush, forsampling the electrical potential of the brush as the developer mixturecarried by the brush periodically contacts portions of the recordingelement between successive charge patterns, said potential resulting, atleast in part, from the triboelectric interaction between the recordingelement and the developer mixture and being inversely proportional tothe concentration of toner particles in the developer mixture.
 2. Theinvention accordance to claim 1 wherein said apparatus further comprisesmeans operatively coupled to said sampling means for activating a tonerreplenishing device when the sampled potential exceeds a predeterminedthreshold level.
 3. The invention according to claim 1 furthercomprising an integrating means operatively coupled with said samplingmeans for accumulating the electrical potential sampled thereby, andmeans for activating a toner replenishing device when the potentialaccumulated by said integrating means exceeds a predefined thresholdlevel.